1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser, especially for lasing in a comparatively short wavelength, for example, capable of lasing in short wavelength, for example, 720 nm, at room temperature for a long liftetime.
2. Prior Art
Semiconductor lasers, for example, III-V compound semiconductor lasers utilizing GaAs-GaAlAs heterostructure formed by liquid phase epitaxial growth have such advantages as they can operate with a high efficiency even with a small sized device, and can be modulated directly by its driving current. Therefore the semiconductor lasers can be expected to have a large prosperous market as sources for various technology of handling light information. In order to utilize the laser for such purpose, it is required to realize a long lifetime. Recently, developments of semiconductor lasers for lasing at the wave lengths shorter than 800 nm are carried on mainly in the GaAlAs lasers. The problem in such laser for lasing in a short wavelength is the shortness of life time due to high contents of Al required for the short wavelength lasing.
FIG. 1 is a sectional view by a sectional plane perpendicular to a direction of laser output light of one example of a conventional visible light semiconductor laser. In the laser of FIG. 1, an active region of Ga.sub.1-x Al.sub.x As (0&lt;x&lt;1) is disposed between a lower clad layer 2 and an upper clad layer 4, both of Ga.sub.1-y Al.sub.y As (0&lt;y&lt;1). In order to obtain a short wavelength of lasing, the value x must be increased, and on the other hand, the value y is selected in a manner to give such a sufficient band gap between the active region 3 and the clad layer 2 or 4 as to confine carriers in the active layer. For example, in order to obtain lasing wavelength of 750 nm, these values are selected that x.perspectiveto.0.18 and y.perspectiveto.0.61, and appropriate thickness of the clad layers 2 and 4 are 1 to 3 .mu.m. By suitably selecting the values x and y, a continuous wave lasing at room temperature becomes possible. However, on the other hand, the increases of the values x (i.e., increase of aluminum content in the active region) induces an increase of stress in the active region, which stress causes a shortened lifetime of the laser.
The stress in the active region is elucidated more in detail. The GaAs which forms substrate 1 has a larger thermal expansion coefficient than GaAlAs which forms the clad layers 2 and 4, and the active region 3 has a thermal expansion coefficient intermediate of the abovementioned two members 1 and 2. The semiconductor laser is in general made by the liquid phase epitaxial growth process, where the growth is made at such a high temperature as about 800.degree. C., and the layers of the laser have different thermal expansion coefficients. Therefore, a considerable internal stress is produced by cooling it down from the high temperature of about 800.degree. C. to a room temperature, thereby the stress is induced in the active region 3.
The larger the difference of the thermal expansion coefficients of the substrate 1 and the active region 3 is, the larger the internal stress of the active region 3 becomes. And the larger the value x (AlAs mole fraction in the active region) is, the larger the difference of the thermal expansion coefficient becomes. And the problem is that in order to obtain lasing with a shorter wavelength in the visible ray range, the wavelength becomes shorter and a larger value of x of the mixed crystal Ga.sub.1-x Al.sub.x As is required. This means that, in the conventional laser of the construction of FIG. 1, the stress in the active region becomes larger when the wavelength of lasing becomes shorter, and this fact has been confirmed both by experiments and calculation. The stress in the active region of the laser of FIG. 1 with respect to the value x varies as shown by a graph of FIG. 2. The details of the conventional laser, characteristic of which is shown in FIG. 1, is as follows:
substrate 1 of n-GaAs . . . 100 .mu.m thick, PA1 first clad layer 2 of n-Ga.sub.1-y Al.sub.y As (0&lt;y&lt;1) . . . 2.5 .mu.m thick, PA1 active region 3 of n-Ga.sub.1-x Al.sub.x As (0&lt;x&lt;1) . . . 0.1 .mu.m thick, PA1 second clad layer 4 of p-Ga.sub.1-y Al.sub.y As (0&lt;y&lt;1) . . . 1.0 .mu.m thick, PA1 electrode contacting layer 5 of p-GaAs . . . 1.0 .mu.m thick, PA1 hetero-isolation layer 6 of n.sup.+ -GaAlAs . . . 1.0 .mu.m thick, PA1 metal electrodes 7 and 8 are . . . on top and bottom faces.
Accordingly, elimination of the stress in the active region 3 due to the difference of the thermal expansion coefficient of the substrate 1 and the active region 3 of the laser is a very important problem in order to improve the lifetime of the semiconductor laser.